Influence of Charged Site Density on Local Electric Fields and Polar Solvent Organization at Oxide Interfaces

The characteristics of oxide surfaces such as their hydroxyl density, the associated acid–base chemistry, and the resulting surface charge play a crucial role in modulating the solvent organization and electrostatics at interfaces found in chemical and environmental processes. Here, the nitrile mode of acetonitrile, a neutral Stark-active molecule, is used to probe the local electric fields and solvent organization that result from surface charge at the α-Al2O3(0001)/aqueous and SiO2/aqueous interfaces using vibrational sum frequency generation (vSFG). The vSFG response in the C≡N stretch region for H2O–acetonitrile mixtures displays an asymmetric line shape and unique pH-dependent behavior, which we attributed to interference with the H2O combination (bend + libration). Stark spectroscopy results at both interfaces reveal that the density of surface hydroxyl groups influences the magnitude of the local electric field experienced by acetonitrile. While the nitrile group of acetonitrile probes single-point charges at the SiO2 surface at pH 6–11, the local electric field sampled by the nitrile group at the Al2O3 interface is impacted by multiple charged hydroxyl sites at elevated pH. These findings highlight the influence of inhomogeneous surface-charging behavior on interfacial solvent structure and local electric fields.